DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 7-8/2018 str. 31     <-- 31 -->        PDF

collected at 8 permanent research plots (PRP) in the basin of the creek in the Belianska Valley. All PRPs represented natural unmanaged riparian stands dominated by grey alder (Alnus incana (L.) Moench.). European ash (Fraxinus excelsior (L.)), sycamore maple (Acer pseudoplatanus (L.)) and black alder (Alnus glutinosa (L.) Gaertn.) are associated with the grey alder stands in the lower parts of the valley (forest plant communities Alnetum incanae and Fraxineto-Alnetum). The predominant soil type was Fluvisol formed on the clayey or gravel alluvial deposits. According to the altitudinal gradient (500-700 m), the Belianska Valley is characterised by moderately warm to cool mountainous climate (Köppen climate classification) with average July temperature 10–17°C, average January temperature from -3 to -90C, precipitation during the growing season 450-650 mm and precipitation during the winter season 300-350 mm. A number of days with snow cover is 120-140 (Lukniš et al., 1972).
Data collection and analyses – Prikupljanje i analiza podataka
Data were conducted in 2015 after the growing season. Series of circular PRPs (P1-P8 (P1: 48°57´39.65´´ N, 19°05´26.43´´ E, P8: 48°59´55.20´´ N, 19°00´17.97´´ E)) wereas designed to capture the entire river basin (Figure 1). According to the altitudinal gradient and incidents of grey alder populations, PRPs were established at an altitudinal step of 20-30 m. The lowest located plot P8 was at the altitude of 525 m and the highest located P1 at 705 m. It was the highest incidence of grey alder in the investigated basin. Centres of circular PRPs covering an area of 150 m2 with radius of 7.5 m were situated 10-14 m from the bank of the creek depending on the alluvial width at an inclination of 2-4%.
In each sample plot, diameter at the breast height (dbh) and the tree height (h) were measured for all standing trees with dbh ≥ 8 cm. Other variables (basal area increment, volume increment) were calculated for each plot and expressed per hectare. At each plot, increment cores were extracted from three co-dominant trees with dbh nearest to the mean stem (Fabrika and Pretch, 2011), at the breast height by the Pressler borer, perpendicularly to the trunk axis. All increment samples were processed by standard dendrochronological procedures (Cook and Kairiukstis, 1992). Samples were scanned by the Epson Expression 10,000 XL scanner. Ring widths were measured to the nearest 0.001 mm using the WinDENDROTM software. Since the age of individuals did not exceed 50 years (relatively short tree-ring series), ring-width series could be cross-dated by the Skeleton plotting technique (Cropper, 1979) using WinDENDROTM software. The output from ARSTAN (Cook, 1985) summarised descriptive statistics for final chronologies of all PRPs (row site chronologies) and whole basin (raw and standard chronology). Tree-ring width series were detrended by the Hugershoff growth curve (procedure of growth trend removal). In order to identify climate factors controlling the tree growth, correlations between standard chronology and mean monthly climate characteristics were computed for the period 1969–2015. The correlations were performed in the 19-month window, from the previous April to the current October. The climate of this period is one of the major influences on the radial increment in each particular year (Rybníček et al., 2012). Monthly precipitation sums and average monthly temperatures were obtained from CRU TS3.21 (0.5 x 0.5 grid interpolated points) available at the KNMI Climate Explorer (Harris et al. 2014; http://climexp.knmi.nl).
To eliminate variability in the growth due to the influence of different thickness, the increment of the basal area (BAI) was calculated for each individual (Phipps and Whiton, 1988).
               
t - age of the tree, rt – tree radius et age t, wt – tree-ring width in age t, BAIt – basal area increment in age t
From all BAIt values in the stand, the average basal area increment (BAIav) and the value of its standard deviation (sd) were calculated. For the calculation of the average stand basal area increment (BAIs), 530 dominant and codominant individuals were considered. This value was the average frequency of trees from the range 490–570 pcs per ha. For the calculation of the average annual volume increment of the stand (iv) the BAIs values were multiplied by the average heights of individuals in PRPs and reduced by the form factor. In the area of the study form factor of grey alder present the value 0.44. The differences in biometric characteristics of PRPs were tested by one-way analysis of variance followed by Duncan’s test.
Radial increments homogeneity (Py) was evaluated by the Pointer year’s method (Schweingruber, 1996). It is an accepted method of showing annual growth reactions due to abrupt changes in the environmental conditions especially due to climatic variations.
               
wt – tree ring width at age t, sx - standard deviation of tree ring widths in the selected period (in this case 5 years)
The pointer years are identified as years that refer to the extreme growth response. For specific pointer year identification, the percentage of trees that revealed either positive or negative growth response in the studied year was taken into consideration. The level of the threshold depends on the aim